Simon Peyton Jones authored Feb 12, 2020 and Marge Bot committed Mar 17, 2020

This patch makes mapType use the successful idiom described
in TyCoRep
   Note [Specialising foldType]

I have not yet changed any functions to use mapType, though there
may be some suitable candidates.

This patch should be a no-op in terms of functionality but,
because it inlines the mapper itself, I'm hoping that there may
be some modest perf improvements.

Metric Decrease:
    T5631
    T5642
    T3064
    T9020
    T14683
    hie002
    haddock.Cabal
    haddock.base
    haddock.compiler